Electrokinetic pumps are capable of delivering fluid in the sub-microliter per minute range at pressures in excess of 5000 psi and thus have been found to be useful for fluid dispensing/metering applications in microfluidic devices, high pressure liquid chromatography systems and micro-analysis systems generally.
An electrokinetic pump comprises an apparatus for converting electric potential to hydraulic force. An electrokinetic pump, such as described in U.S. Pat. Nos. 6,013,164 and 6,019,882 to Paul and Rakestraw, typically consists of at least one duct or channel, that can be a capillary channel or microchannel that forms a fluid passageway having an inlet and an outlet. The capillary duct or channel contains an electrolyte and has a porous stationary phase or substrate typically comprising a nonporous dielectric medium disposed therein between one or more pairs of spaced electrodes. The porous stationary phase can include small nonporous particles, high surface area structures fabricated within the microchannel, or microporous materials such as monolithic polymer networks. An electric potential is applied between the spaced electrodes in contact with electrolyte, or pump fluid, that can be an aqueous or an organic liquid or mixtures thereof, to cause the electrolyte to move in the microchannel by electroosmotic flow and generate a pressure whose magnitude depends on the Darcy permeability of the fluidic channels downstream of the pump. Pump performance in terms of pressure generated per volt of applied electric potential is determined by composition of the porous dielectric material, the composition of the stationary phase and geometry as well as the properties of the electrolyte.
At the interface between a charged solid and an electrolyte solution an electrochemical double layer is formed and the mobile (diffuse) component of the double layer moves in response to the force generated by an externally applied electric field giving rise to electroosmotic flow.
It has long been recognized in the separations art that in capillary-based devices the zeta (ζ) potential plays a strong role in consistency of electroosmotic flow velocity and the consequent effect on separation efficiency as a result of nonuniform flow in capillary channels (Liu et al. Anal. Chem., 2000, 72, 5939-5941. In the case of electrokinetic pumps, the material comprising the capillary channel walls affects the ζ potential and maximizing the ζ potential will maximize pressure, flow rate, and pump performance. Silica surfaces have high wall ζ potentials at neutral pH and above, and are a common material choice. However, these high energy surfaces can interact with and adsorb many compounds, notably bases. Moreover, the use of silicon dioxide materials in separation systems is further restricted due to the chemical stability of these substrate materials. At pH values greater than about 7 the dissolution rate of silicon dioxide materials increases due to the general weakness of the Si—O—Si bond.
In an attempt to lower costs and overcome the disadvantages of glass and quartz microchips as well as improve electroosmotic flow and thus improve separations efficiency, a variety of different capillary channel materials has been proposed such as polystyrene, poly-(ethylene terphthalate glycol) and fluorocarbons. In addition, a wide variety of channel wall coatings, such as zirconia particles, sulfonic acid groups bonded to silica and quarternary amine groups bonded to silica have been created (Reyes et al., Anal. Chem., 2002, 74, 2623-2636 and references cited therein). However, these channel wall surface modifications display problems such as limited lifetimes, surface contamination, surface charge neutralization and complex fabrication methods. Moreover, many of the surface modifications either degrade under extreme conditions such as very high or low pH or only operate effectively over a limited pH range.
Prior attempts to fabricate positively charged pumping media with zirconia particles, and sulfonic acid groups or quaternary amine groups bonded to silica channel surfaces have failed to produce reliable and robust pumps. Each display problems with limited lifetime, surface contamination, surface charge neutralization, and fabrication complexity.
Polymer monolithic materials have been employed as the stationary phase in electrokinetic pumps, cf. U.S. patent application Ser. No. 09/796,762, Castable Three-dimensional Stationary Phase for Electric Field-driven Applications, filed Feb. 28, 2001 now U.S. Pat. No. 6,846,399.These materials are potentially attractive as stationary phase materials, in particular, because they can be fabricated with charged sites within the polymer structure, thereby providing desired the ζ potential. However, it has been found that EK pumps fabricated with polymer monoliths containing quaternary amine (NR4+) charged groups can suffer from efficiency losses likely due to relatively low surface charge density and ζ potential, an overly broad distribution of pore sizes, lesser structural integrity of the polymer relative to more robust stationary phase materials such as silica, and damage by capillary forces if the polymer monolith is dried after fabrication, and combinations thereof.